Optical instruments

ABSTRACT

A wide angle, low focal ratio, high resolution, catoptric, image plane line scanner which embodies three interrelated main features, a reflective improvement on the Schmidt principle, a polar line scanner in which all field elements are brought to and corrected on axis, and a scanner arrangement in which the aperture stop of the system, located in a relay unit, is effectively imaged at the center of curvature of a spherical primary mirror in place of the physical stop unit in the classical Schmidt configuration, making the system consequently much more compact. The system scans at a large radial angle and an extremely high rate of speed with relatively small scanning mirrors, and since it is symmetrical about the optical axis, the obscuration is independent of scan angle.

v lax-my Liv-av Abel Jan. 1, 1974 [5 OPTICAL INSTRUMENTS 3,455,6237/1969 Harris 350/27 [75] Inventor: Irving Raymond Abel, Lexington,

Mass Primary Examiner-David Schonberg Assistant ExaminerConrad Clark[73] Assignee: The United States of America as Atmmey Marvin J Mamock etaL represented by the Administrator of the National Aeronautics andSpace Administration, Washington, DC.

[22] Filed: Apr. 10, 1972 [57] ABSTRACT A wide angle, low focal ratio,high resolution, catoptric, image plane line scanner which embodiesthree 1 pp 242,662 interrelated main features, a reflective improvementon the Schmidt principle, a polar line scanner in which 52 US. Cl356/216, 356/43, 350/7, field elements are bmught and muted axis 350/285250/236 and a scanner arrangement in which the aperture stop 51 Int. ClG01j 1/56, G01 j 5/48 of the System located in a relay "nit, iseffectively [58] Field of Search 350/6, 7, 22, 23, aged at the Center ofcurvature of a Spherical Primary 350/27 55, 285 273 275, 288, 289;mirror in place of the physical stop unit in the classical 17 1 24, 25,configuration, making the system consc- 43 quently much more compact.The system scans at a large radial angle and an extremely high rate ofspeed [56] References Cited with relatively small scanning mirrors, andsince it is UNITED STATES PATENTS symmetrical about the optical axis,the obscuration is independent of scan angle.

3,443,853 5/1969 Todd, Jr. .1 356/216 2,873,381 2/1959 Lallroesch 350/713 Claims, 3 Drawing Figures I4 1| s y/F oatm: 56/216 G P'ATENTEDJA'N HmI 3782 835 SHEET 1 BF 2 FIG.I

PATENTED 11974 3,782,835

sum a nr 2 OPTICAL INSTRUMENTS BACKGROUND OF THE INVENTION Thisinvention relates generally to the field of optical instruments, andmore particularly to scanning radiometers and spectroradiometers. Suchinstruments are used for example in earth resources surveys made fromhigh altitude vehicles and satellites using radiation in theultraviolet, visible, and infrared bands of wavelengths.

ORIGIN OF THE INVENTION In order to be useful such surveys must coversignificant ground area with adequate resolution: a rapid scan rate isnecessary, which means that the optical system must have a low focalratio and that moving masses must be kept as small as possible.Monochromatic defects must be minimized, chromatic aberrations must beavoided, and any obscuration must be independent of scan angle. Thiscombination of restraints presents a very difficult overall problem,which the present invention was designed specifically to solve.

SUMMARY OF THE INVENTION The principal object of the invention isaccordingly to provide an improved scanning radiometer. Anotherprincipal object of the invention is to provide an improved catoptricoptical system. Other objectives of the invention are to provide animproved scanning arrangement, to provide catoptric means for minimizingaberrations in an optical system, and to provide an image plane scanningcatoptric optical system in which obscuration is independent of scanningangle.

A more detailed object of the invention is to provide a wide angle, highspeed, low focal ratio, high resolution, catoptric image plane linescanner of compact dimensions with a rapid scan rate, a large scanangle, a small moving mass, and an obscuration which is independent ofscan angle.

Various other objects, advantages, and features of novelty whichcharacterize my invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and objects attained byits use, reference should be had to the drawing which forms a furtherpart hereof, and to the accompanying descriptive matter, in which thereare illustrated and described certain preferred embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1, is a partially schematicvertical longitudinal section of an optical system embodying myinvention;

FIG. 2 shows the obscuration present in the system of FIG. 1; and

FIG. 3 shows a modification of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, thepreferred embodiment of my invention is shown to comprise a housingthrough which passes the optical axis 11. Radiation entering the housingat an opening 12 is transmitted through a catoptric optical system 13 toa utilization device or detecting means 14 which includes an infrareddetector or an array of such detectors, and may also bly 23, and relayoptics 24 comprising a housing 25 1 containing a spherical mirror 26 andan aspherical mirror 27. Mirrors 21, 26, and 27 have central aperturesand their axes define axis 11. Mirror 27 acts as the aperture stop ofthe system, and is imaged at the center of curvature of mirror 21 as theentrance pupil of the system.

sann e 1x2 p p ,P .mjr..- rors 30 5116 31 mo untedifof fii'itaiijfiti nabout axis ll,w hi li p'aes through mirror 30 at aii a cute angle. Thesemirrors are normal to a common plane containing axis 11 and formopposite faces of a rhomboidal polyhedron. Upon rotation of assembly 23mirror 31 traces a conical surface coaxial with axis 11.

A mask 38 is mounted between secondary mirror 22 and scanner assembly23, and has an arcuate aperture or slot 39 through which the convergingbeam from the secondary mirror may reach the scanner. The purpose ofthis mask is to cut off from the scanner assembly radiation for thetrailing half of the field being obscured, and the angular extent ofslot 39 is selected accordingly. If desired one or more calibrationsources of radiation may be mounted on mask 38 in the unslotted portionthereof. At each rotation of the scanner assembly radiation for each ofsuch sources would reach detector 47 so that system operation may becontinuously monitored.

As an alternative to mask 38 it is possible to interrupt the electriccircuit from detecting means 14 in synchronism with the rotation ofscanner assembly 23 during the trailing half of each scan.

Mirror 22 is so located, in relation to mirror 21 that parallel lightentering the system is focused in a plane perpendicular to axis 11, andmirror 31 continuously intersects this plane in every rotated portion ofthe scanner assembly. Radiation entering the instrument in a directionhaving a major component aligned with axis 11 emerges from scannerassembly 23 as a diverging axial beam which enters relay optics 24through the aperture in reflector 26. This beam is reflected from mirror27 to mirror 26 and thence as a converging axial beam through theaperture in reflectors 27 and 21 to detecting means 14.

Mirror 27 is basically a spherical mirror, but is figured or modified tocorrect not only for the spherical aberration introduced to the systemby mirror 21, but also for that introduced by mirror 26 and due to itsown basic spherical characteristics. The concept of catoptricallycorrecting for the spherical aberration introduced by a sphericalreflector is one of the significant contributions of my invention: itdiffers from the dioptric correction of the classical Schmidt system inthat it is equally effective for all wavelengths over a wide range,whereas a refractive correction plate is wavelength sensitive. Theimproved arrangement more exactly produces the effective on-axiscondition for all off-axis field points relative to the sphericalprimary mirror.

As in any catoptric system a considerable portion of the entrance pupilis obscured by optical elements. FIG. 2 shows the obscurations due tothe aperture in mirror 21, to mirror 22, and to relay optics 24, whichare seen to largely overlap. The useful area of the entrance pupil atany one instant is the generally crescent shaped area above theobscurations in FIG. 2. As scanner assembly 23 rotates, the obscurationpattern rotates about the center of the entrance pupil so that there isno variation of obscuration with scan angle. This is due to the factthat the system is essentially symmetrical about the axis 11.

Because the path of mirror 31 lies in the focal plane of mirror 21 theformer need only be of small size. This minimizes the moving mass of thesystem, and the scanner assembly operates without difficulty at highspeed.

The path of one marginal ray through the optical system may be tracedthrough points 40, 41, 42, 43, 44, 45, 46, and 47. The path of theopposite marginal ray may be traced through points 50 and 51 to point52, where it is obscured by relay optics housing 24: in the absence ofthe obscuration the ray would continue through points 53, 43, 54, 55,56, and 47.

It will be clear that operation of scanning assembly 23 is effective toconvert the scan of an arcuately related succession of field elements inthe image plane of mirror 21 into rotation of the beam from the scannerabout axis 11, accompanied by apparent transverse movement of the fieldacross the beam, the instantaneous intensity of the beam being that ofthe field element at each rotated portion of assembly. In other wordsthere is provided a polar line scan in which all field elements arebrought to the axis and corrected by mirror 27.

In use the instrument is mounted in a suitable vehicle such as anorbiting satellite, so arranged that optical axis 11 is directed towarda field to be scanned. Movement of the vehicle in its orbit carries theoptical axis across the field. Rotation of scanner assembly 23 iseffective to produce an arcuate linear sweep generally transverse to thedirection of movement of the vehicle, its extent being limited by mask38. By the time the scanner assembly has completed a revolution thevehicle has advanced with respect to the field so that the nextoccurring linear sweep is displaced laterally with respect to theprevious sweep, and a continuous series of sweeps results in completecoverage of the area in question.

By way of illustration if the instrument described above were directeddownwardly from a satellite at an altitude of 100 nautical miles and aresulting orbital period of 90 minutes for an earth circuit of 25,000miles, successive sweeps of earths surface transverse to the path of thesatellite would be spaced along its ground path by roughly 260 feet.

In the embodiment of the invention shown in FIG. 1 the entrance pupildiameter was 17 inches, of which about 47 percent was subject toobscuration. The radii of curvature of mirrors 21 and 26 were 40.8inches and 25.96 inches respectively. Mirror 27 was deformed from aradius of curvature of 15.163 inches. Mirrors 30 and 31 made angles of41.25 and 385 respectively with planes normal to the axis, mirror 31 hada transverse diameter of 0.245 inches and rotated at a radius of 1.984inches from axis 11. Slot 39 extended for about 120 around mask 38.Parallel light entering the instrument at an angle of 5.5 with respectto the axis emerged from the scanner axially, and the scanner assemblyrotated at 6,000 revolutions per minute.

FIG. 3 shows a second embodiment of the invention. Here sphericalprimary mirror 21, plane secondary mirror 122 and scanning assembly 123resemble their counterparts in FIG. 1. Relay optics 124 includesspherical mirror 126 and an aspherical mirror 127 is also generally thesame, but is located on the opposite side of mirror 12] from assembly123. The converging beam from relay optics 124 is directed by a foldingmirror 128 to a field stop 129, behind which may be located any desireddetecting means with or without an initial spectrometer. In the Figureit is suggested that elements 121, 122, and 123 are located outside theshell 138 of a vehicle, and that the remaining elements are located in aclosed chamber 139 inside the vehicle.

It will be apparent that this embodiment of the invention includes thesame reflecting correction for spherical abberration, the sameconversion of arcuate to rotational scanning, and the same relay mirrorexpedient as the embodiment first described.

Numerous objects and advantages of my invention have been set forth inthe foregoing description, together with details of the structure andfunction of the invention, and the novel features thereof are pointedout in the appended claims. The disclosure, however, is illustrativeonly, and changes may be made in detail, especially in matters of shape,size, and arrangement of parts, within the principle of the invention,to the full extent indicated by the broad general meaningof the terms inwhich the appended claims are expressed.

I claim as my invention:

1. A scanning radiometer comprising, in combination:

input optics having an optical axis and arranged to create, in an imageplane generally transverse to said axis, a primary image of theradiation from a field of view;

a scanner assembly rotatable about said axis for convetting the scan ofan arcuately related succession of field elements in said plane intorotation of a beam directed generally along said axis accompanied byapparent transverse movement of the field across the beam, theinstantaneous intensity of said beam being that of the image fieldelement at each particular rotated position of said assembly; radiationresponsive means;

and relay optics for focusing said beam from said assembly on saidradiation response means.

2. Apparatus according to claim 1 in which said input optics includes aspherical element and said relay optics includes means for correctingsaid beam for spherical aberration introduced by said spherical element.

3. Apparatus according to claim 1 in which said input optics includes aspherical mirror and said relay optics includes catoptric means forcorrecting said beam for aberrations introduced by said sphericalmirror.

4. Optical apparatus comprising, in combination:

rotation about an axis and including a first mirror through which saidaxis extends at an acute angle and a second mirror spaced from androtatable unitarily with said first mirror so that upon rotation of saidassembly said second mirror traces a surface which is a frustum of aright circular cone coaxial with the axis of rotation, said mirrorsbeing normal to a common plane containing said axis;

input optics having an axis coincident with that of said scannerassembly and arranged to create a primary image of an off-axisinstantaneous field in an image plane intersecting said frustum, theangular relation between said mirrors being such that radiation fromsaid primary image is reflected by said mirrors sequentially in a beamdirected along said axis regardless of the rotated position of saidassembly, the area of said second mirror projected on said image planebeing comparable to the size of said field of view at said image plane;

and means mounted to receive said beam for giving an output determinedthereby, so that as said scanner assembly rotates said output isrepresentative of the intensity of the radiation from arcuately relatedseries of portions of said image plane.

5. Apparatus according to claim 4 in which said input optics includes aspherical element;

and relay optics mounted on said axis for receiving said beam andcorrecting it for spherical aberration introduced by said sphericalelement.

6. Apparatus according to claim 4 in which said input optics includes aspherical mirror and relay optics mounted on said axis for receivingsaid beam, and including catoptric means for correcting for sphericalaberration introduced by said spherical mirror.

7. A wide field catoptric optical system having an entrance pupil andcomprising, in combination:

a primary spherical mirror having a central aperture;

a plane secondary mirror;

an initial plane scanning mirror rotatable about an axis externalthereto which makes an acute angle with the plane thereof;

another plane scanning mirror rotatable about said axis, which passestherethrough and makes an acute angle with the plane thereof;

means causing simultaneous rotational motion of said scanning mirrors;

a first relay mirror mounted on said axis and having a central aperture;

a second relay mirror mounted on said axis and having a centralaperture;

and radiation responsive means mounted on said axis;

said mirrors being positioned so that radiation passing said entrancepupil in a direction having a major component parallel to said axis isreflected from said primary spherical mirror and said plane secondarymirror to focus at a primary image plane which said initial scanningmirror continuously intersects at an acute angle, so that radiation fromsaid image plane is serially reflected by said scanning mirrors as abeam emerging coaxial with said axis, and so that said beam passesthrough all said central apertures and is focused by said relay mirrorson said detector means, the central aperture of one of said relaymirrors comprising the aperture stop of the system.

8. Apparatus according to claim 7 in which one of said relay mirrors isfigured to correct the spherical aberration introduced by all saidspherical mirrors.

9. Apparatus according to claim 7 in which said relay mirrors arebetween said scanning mirrors and said primary spherical mirrors.

10. Apparatus according to claim 7 in which said primary sphericalmirror is between said scanning mirrors and said relay mirrors.

11. Apparatus according to claim 8 in which said beam falls on saidfigured relay mirror for reflection to the other relay mirror.

12. Apparatus according to claim 7 in which the dihedral angles of saidscanning mirrors are such that radiation entering said pupil at an angleof 5 /11" with respect to said axis emerges from said second scanningmirror as a beam aligned with said axis, for all rotated positions ofsaid scanning mirrors.

13. Apparatus according to claim 7 in which the center of curvature ofsaid primary spherical mirror is in the plane of said entrance pupil.

1. A scanning radiometer comprising, in combination: input optics havingan optical axis and arranged to create, in an image plane generallytransverse to said axis, a primary image of the radiation from a fieldof view; a scanner assembly rotatable about said axis for converting thescan of an arcuately related succession of field elements in said planeinto rotation of a beam directed generally along said axis accompaniedby apparent transverse movement of the field across the beam, theinstantaneous intensity of said beam being that of the image fieldelement at each particular rotated position of said assembly; radiationresponsive means; and relay optics for focusing said beam from saidassembly on said radiation response means.
 2. Apparatus according toclaim 1 in which said input optics includes a spherical element and saidrelay optics includes means for correcting said beam for sphericalaberration introduced by said spherical element.
 3. Apparatus accordingto claim 1 in which said input optics includes a spherical mirror andsaid relay optics includes catoptric means for correcting said beam foraberrations introduced by said spherical mirror.
 4. Optical apparatuscomprising, in combination: a double folding scanner assembly arrangedfor rotation about an axis and including a first mirror through whichsaid axis extends at an acute angle and a second mirror spaced from androtatable unitarily with said first mirror so that upon rotation of saidassembly said second mirror traces a surface which is a frustum of aright circular cone coaxial with the axis of rotation, said mirrorsbeing normal to a common plane containing said axis; input optics havingan axis coincident with that of said scanner assembly and arranged tocreate a primary image of an off-axis instantaneous field in an imageplane intersecting said frustum, the angular relation between saidmirrors being such that radiation from said primary image is reflectedby said mirrors sequentially in a beam directed along said axisregardless of the rotated position of said assembly, the area of saidsecond mirror projected on said image plane being comparable to the sizeof said field of view at said image plane; and means mounted to receivesaid beam for giving an output determined thereby, so that as saidscanner assembly rotates said output is representative of the intensityof the radiation from arcuately related series of portions of said imageplane.
 5. Apparatus according to claim 4 in which said input opticsincludes a spherical element; and relay optics mounted on said axis forreceiving said beam and correcting it for spherical aberrationintroduced by said spherical element.
 6. Apparatus according to claim 4in which said input optics includes a spherical mirror and relay opticsmounted on said axis for receiving said beam, and including catoptricmeans for correcting for spherical aberration introduced by saidspherical mirror.
 7. A wide field catoptric optical system having anentrance pupil and comprising, in combination: a primary sphericalmirror having a central aperture; a plane secondary mirror; an initialplane scanning mirror rotatable about an axis external thereto whichmakes an acute angle with the plane thereof; another plane scanningmirror rotatable about said axis, which passes therethrough and makes anacute angle with the plane thereof; means causing simultaneousrotational motion of said scanning mirrors; a first relay mirror mountedon said axis and having a central aperture; a second relay mirrormounted on said axis and having a central aperture; and radiationresponsive means mounted on said axis; said mirrors being positioned sothat radiation passing said entrance pupil in a direction having a majorcomponent parallel to said axis is reflected from said primary sphericalmirror and said plane secondary mirror to focus at a primary image planewhich said initial scanning mirror continuously intersects at an acuteangle, so that radiation from said image plane is serially reflected bysaid scanning mirrors as a beam emerging coaxial with said axis, and sothat said beam passes through all said central apertures and is focusedby said relay mirrors on said detector means, the central aperture ofone of said relay mirrors comprising the aperture stop of the system. 8.Apparatus according to claim 7 in which one of said relay mirrors isfigured to correct the spherical aberration introduced by all saidspherical mirrors.
 9. Apparatus according to claim 7 in which said relaymirrors are between said scanning mirrors and said primary sphericalmirrors.
 10. Apparatus according to claim 7 in which said primaryspherical mirror is between said scanning mirrors and said relaymirrors.
 11. Apparatus according to claim 8 in which said beam falls onsaid figured relay mirror for reflection to the other relay mirror. 12.Apparatus according to claim 7 in which the dihedral angles of saidscanning mirrors are such that radiation entering said pupil at an angleof 5 1/2 * with respect to said axis emerges from said second scanningmirror as a beam aligned with said axis, for all rotated positions ofsaid scanning mirrors.
 13. Apparatus according to claim 7 in which thecenter of curvature of said primary spherical mirror is in the plane ofsaid entrance pupil.